Basis of Presentation - Narrative (Detail)	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($) reporting_unit	Dec. 31, 2017 USD ($) reporting_unit
Basis Of Presentation [Line Items]
Number of reporting units \| reporting_unit	3	4
Goodwill, impairment loss	$ 0
Percentage of goodwill allocated to reporting units	92.00%
Goodwill	$ 4,573,000,000	$ 4,535,000,000
Arconic Engines
Basis Of Presentation [Line Items]
Goodwill	2,095,000,000
Alcoa Fastening Systems And Rings
Basis Of Presentation [Line Items]
Goodwill	1,623,000,000
Arconic Engineered Structures
Basis Of Presentation [Line Items]
Goodwill	$ 517,000,000

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13777-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration